Very interesting question Bram, and one i am sure will raise some serious debate.
I am not with my books right now, but i believe Stone makes reference to learning keris fighting moves from a prince of Jawa. Though some of his keris info is suspect, this reference actually names the prince and i can't see why he would make it up. Early Chinese contact brought back reports of the Javanese using the keris as a weapon back in the 15th century. People have argued these reports because the Chinese also made derogatory remarks about the Javanese as well. I am not sure that those remarks should necessarily negate the substance of the weapon use reports however. It does seem to me that the keris was at least used as a weapon in Jawa at one time. The exact point at which that changed may be very difficult to track.